As disclosed in “Report on the national general survey of soil contamination” issued in 2014 by Ministry of Environmental Protection of the People's Republic of China and Ministry of Land and Resources of the People's Republic of China, the cadmium contaminated soil area accounts for 7.0% in China, therefore the treatment of the cadmium contamination in soil is a serious environmental problem to be solved in China, wherein the cadmium contamination is particularly prominent in rice field.
The treatment technologies against the cadmium contamination in rice field, are roughly divided into two classes on the basis of the treatment objectives, a remediation technology of removal of the heavy metals in soil, which has an objective for making the soil quality meet the environmental standards, and a contamination control technology, which has an objective for guaranteeing the safety of agricultural products.
At present, it has been confirmed that the cadmium hyper-accumulated plants mainly comprise Sedum alfredii Hance, sedum plumbizincicola, Solanum nigrum L., Thlaspi arvense L., and the like, and the cadmium enriched plants comprise Amaranthus hypochondriacus L, and the like. The plant remediation technology requires to occupy farmland and farming time, requires a large investment of funds, therefore it is hard to be applied in a large area. At present, the plant remediation technology is only suitable for dry lands, and there are no reports on application of the same in the flooded rice field yet. In addition, there are no one-to-one correlation between the heavy metal contents in soil and the heavy metal contents in agricultural products, therefore reduction of the heavy metal contents in soil to meet the standard level can not guarantee that the safeties of the agricultural products meet the standard levels.
Obviously, the remediation of the cadmium contamination in rice field is difficult and presents many technical bottlenecks, therefore the removal technique is difficult to solve the cadmium contamination in the regional rice field. It is a practical and feasible technical approach for improving the quality of the agricultural products by reducing the bioavailability of cadmium, preventing the absorption and accumulation of cadmium in rice, particularly inhibiting the transportation of cadmium from the rice leaves to the grains.
A new technical idea for controlling the heavy metal contamination in farmlands is disclosed in Chinese Patent Nos. 200610036994.8, 201010156359.X, 201010156358.5, 201310737996.X, etc.: the composite silica sol is directly sprayed on the surface of the rice leaves, without treating the soil, so as to effectively reduce the heavy metal contents in the agricultural products.
In technical principles, cadmium is a non-essential element in rice, which can compete for the transport proteins for the essential elements or the beneficial elements, such as Si, Zn, Fe, Mn and the like, so as to enter the roots and be transported upward into the grains (Uraguchi & Fujiwara, 2013). The accumulation of cadmium in rice grains mainly involves in four processes: absorption in the root, transportation in the xylem, transportation across the vascular bundles, and migration from the phloem to the grains (Uraguchi & Fujiwara, 2012). At present, some important genes which are related to the cadmium absorption in rice and their expression products, the transport proteins, have been identified. In the process of the cadmium absorption in the rice roots, the iron transport protein produced by the expression of the constitutive gene OsNramp5 can absorb Mn and Fe, and simultaneously efficiently transport cadmium (Sasaki et al., 2012), the zinc iron transport protein produced by the expression of the genes OsZIP1 and OsZIP3 can transport Zn and Fe, and simultaneously transport cadmium (Ramesh et al., 2003), in addition, the iron transport protein produced by the expression of the Fe-deficiency induction gene OsNramp1, OsIRT1 and OsIRT2 can promote the Fe absorption and simultaneously enhance the cadmium absorption (Takahashi et al., 2011; Nakanishi et al., 2006; Lee & An, 2009). In the process of loading in the xylem and upward transportation, the transport protein produced by the expression of gene OsHMA2 can load Zn in the xylem, and simultaneously can load cadmium (Nocito et al, 2011; Takahashi et al., 2012), the transport protein produced by the expression of genes OsHMA3, OsMTP1, OsABCG43/OsPDR5, etc., can transport the elements, such as zinc, cadmium, and the like, into the vacuoles so as to maintain the equilibrium state of the various elements in the cells, and inhibit the transportation thereof into the xylem (Ueno et al., 2010; Miyadate et al., 2011; Menguer et al., 2013; Oda et al., 2011). In the process of the loading in the phloem and the transportation into the grains, mainly the transport proteins such as OsLCT1 and the like, can regulate the transportation of the heavy metal elements, such as manganese, cadmium, etc., from the phloem into the grains (Uraguchi et al., 2011).
On the basis of the above analysis, the absorption and transportation of the non-essential elements, such as cadmium and the like, in the rice body, can typically be achieved by means of the transport proteins for the essential elements having similar structures, such as Fe, Zn, Si, and the like, and at present, the specific transport protein which can solely transport the heavy metal cadmium has not be found yet. Therefore, on the basis of the characteristics that the heavy metal elements compete for the same transport proteins with the essential elements, the application of the exogenous essential elements can prevent the absorption and in vivo transportation of the heavy metal.
In addition, rice is a typical silicon-philic plant, however, up to now, it is uncertain that silicon is the essential element for plant growth, but there are sufficient evidences showing that silicon is necessary for the rice production in stable and high yield. Silicon is present in the juice in the xylem of the plant, mainly in monosilicic acid state [Si(OH)4], with a small proportion of ionic state Si(H3SiO4−). The ionic state of silicon has a slight high proportion in the rice roots, up to 3%˜8%, dominantly in Si(OH)4 form; the vast majority of the silicon in the rice body is present in the form of hydrated amorphous silica SiO2.H2O or polysilicic acid, accounts for about 90%˜95% of the total silicon content, and a minority thereof is silicic acid, colloidal silicic acid and silicate ions.
The absorption of silicon in rice is a typical active absorption process, and silicon is absorbed in the form of silicic acid in the rice roots, therefore such active transportation makes the silicic acid content in the solution in the rice xylem is many times higher than that in the solution outside the roots. At present, three Si transport genes (OsLsi1, OsLsi2 and OsLsi6) have been successfully cloned from the rice silicon absorption-deficient mutants, wherein the genes OsLsi1 and OsLsi2 are located on chromosomes 2 and 3 respectively, mainly expressed in the roots; the gene OsLsi6 is located on chromosome 6, mainly expressed in the leaf sheaths and blades, and sparsely expressed in the roots (Ma et al., 2006; 2007; Yamaji et al., 200). The processe of the absorption and transportation of silicon in rice involves in 4 steps: in the first step, the silicic acid can be transported from the outer solution into the cells by the transport protein OsLsi1 in the exodermis, and the silicic acid is released into the apoplast of the aerenchyma by the transport protein OsLsi2; in the second step, the silicic acid is transported from the apoplast solution into the endodermis cells by the transport protein OsLsi1 in the endodermis, and the silicic acid is transported into the pericycles by the transport protein OsLsi2; in the third step, the silicon is transported from the pericycles to the above-ground part through the xylem vessel, in the non-polymeric monosilicic acid form, with the transpiration stream; in the fourth step, the transport protein OsLsi6 which is positioned in the xylem parenchyma cells close to the side of the vessels in the leaf sheaths and blades is responsible for unloading and distributing the silicon in the xylem, and dehydrated and polymerized under the transpiration effect, so as to form silica gel (SiO2.nH2O), which is deposited in the cell wall and the intercellular space in the different tissues and organs in the above-ground part, wherein more than 90% of silicon is present, in silica gel form, in rice.
Silicon is an important constituent element in rice body, and also a main constituent component in the cell wall of rice. After deposited in the apoplast (the cell wall and the intercellular space), silicon can not only reduce the cell voidage, but also can co-precipitate with cadmium and bind the same within the cell wall, so as to reduce the transportation of cadmium from the apoplast into the cell (Shi et al. 2005; Shi et al., 2010). Silicon can also improve the oxidative stress resistance in rice, alleviate the cadmium stress toxicity, and promote the increase of biomass (Liang et al., 2007), so that the exogenous application of silicon can improve the biomass of rice, and simultaneously reduce the cadmium content in the grains.
On the basis of the above mechanism of silicon, it is clear that silicon can prevent the accumulation of cadmium in the grains, so that silicon is determined to be used as a main exogenous regulator for preventing cadmium in rice grains. The above four authorized patents are the technologies for preventing the accumulation of cadmium in rice grains using the silica sol as the exogenous regulator, and provide the processes for preparing the silica sol, the rare-earth composite silica sol, the selenium dioxide composite silica sol.
However, the above four patents do not involve in how the silicon and selenium affect the active expression of the cadmium absorption and transportation-related gene in rice, how the foliage blocking agents are applied to achieve the precise regulation of the active expression of the cadmium absorption and transportation-related genes in rice, so as to inhibit the accumulation of cadmium in rice. These patents have not provided the precise regulation technologies and methods on these aspects yet.